In recent years, in order to cope with the air pollution and the global warming, it is sincerely desired that the amount of carbon dioxide be reduced. In the automobile industry, expectations are centered on reduction of an emission amount of carbon dioxide by introduction of an electric vehicle (EV) and a hybrid electric vehicle (HEV). Thus, development of an electrical device such as a secondary battery for driving a motor, the electrical device serving as a key for practical use of these vehicles, is assiduously pursued.
It is required that the secondary battery for driving a motor have extremely high output characteristics and high energy in comparison with lithium ion secondary battery for general use in a cellular phone, a notebook computer and the like. Hence, a lithium ion secondary battery having the highest theoretical energy among all of the batteries has attracted attention, and development thereof is rapidly advanced at present.
In general, the lithium ion secondary battery has a configuration, in which a positive electrode and a negative electrode are connected to each other while interposing an electrolyte layer therebetween, and are housed in a battery case, the positive electrode having a positive electrode active material and the like coated on both surfaces of a positive electrode current collector by using a binder, and the negative electrode having a negative electrode active material and the like coated on both surfaces of a negative electrode current collector by using a binder.
Heretofore, for the negative electrode of the lithium ion secondary battery, a carbon/graphite-based material advantageous in terms of a lifetime of a charge/discharge cycle and cost has been used. However, in the carbon/graphite-based negative electrode material, charge/discharge is performed by occlusion/discharge of lithium ions into/from graphite crystals, and accordingly, there is a disadvantage that a charge/discharge capacity equal to or more than a theoretical capacity of 372 mAh/g, which is obtained from LiC6 that is a maximum lithium-introduced compound, cannot be obtained. Therefore, it is difficult to obtain a capacity and an energy density, which satisfy a practical level of usage for a vehicle, by the carbon/graphite-based negative electrode material.
As opposed to this, in a battery using a material, which is alloyed with Li, for the negative electrode, an energy density thereof is enhanced in comparison with the conventional carbon/graphite-based negative electrode material, and accordingly, such a material is expected as a negative electrode material in the usage for the vehicle. For example, a Si material occludes/discharges 4.4 mol of lithium ions per 1 mol as in the following Reaction formula (1) in the charge/discharge, and in Li22Si5 (=L14.4Si), a theoretical capacity thereof is 2100 mAh/g. Moreover, in a case of calculating such a theoretical capacity per weight of Si, the Si material has an initial capacity of no less than 3200 mAh/g (refer to Comparative reference example 42 of Reference example C).[Chem. 1]Si+4.4Li++e−↔Li4.4Si  (A)
However, in the lithium ion secondary battery using the material, which is alloyed with Li, for the negative electrode, expansion/contraction in the negative electrode at a time of the charge/discharge is large. For example, volume expansion in the case of occluding the Li ions is approximately 1.2 times in the graphite material, and meanwhile, in an event where Si and Li are alloyed with each other, the Si material makes transition from an amorphous state to a crystal state and causes a large volume change (approximately four times), and accordingly, there has been a problem of lowering a cycle lifetime of the electrode. Moreover, in a case of a Si negative electrode active material, a capacity and cycle durability thereof are in a tradeoff relationship, and there has been a problem that it is difficult to enhance the high cycle durability while exhibiting a high capacity.
In order to solve such problems as described above, a negative electrode active material for a lithium ion secondary battery, which contains an amorphous alloy having a formula: SixMyAlz, is proposed (for example, refer to Patent Literature 1). Here, x, y and z in the formula represent atom percent values, x+y+z=100, x≥55, y<22, z>0, and M is metal composed of at least one of Mn, Mo, Nb, W, Ta, Fe, Cu, Ti, V, Cr, Ni, Co, Zr and Y. In the invention described in Patent Literature 1, in the paragraph [0018], it is described that a content of the metal M is minimized, whereby a good cycle lifetime is exhibited as well as a high capacity.